docs: def variance(self) -> Optional[float]:  #, ddof: int = 0) -> float:
          """Statistical variance of all values entered into histogram.
  
          This number is precise, because we keep the necessary data
          separate from bin contents.
  
          Returns
          -------
          float
          """
          # TODO: Add DOF
          # http://stats.stackexchange.com/questions/6534/how-do-i-calculate-a-weighted-standard-deviation-in-excel
          if self._stats:
              if self.total > 0:
                  return (self._stats["sum2"] - self._stats["sum"] ** 2 / self.total) / self.total
              else:
                  return np.nan
          else:
              return None